The interesting thing about menopause evolutionarily is actually the reverse -- why are humans capable of living so long after menopause? Members of most species don't survive very long, if at all, after they're no longer capable of reproducing. Human women can survive even significantly after their youngest offspring reaches adulthood, so it's not just that we have to live long enough for the youngest kid to have a chance of living. Greater survival rate of the grandchildren, maybe?

There is a cost to having older members of a group stick around (food and other resources) but also a benefit, particularly in a social species with a limited range.

First, I really don’t know how we compare to say, a dog or the various kinds of wolf (also social species) in terms of the fraction of one’s life that one can be reproductive. I do believe other great apes (gorillas and such) can survive well past their reproductive prime. If so, and assuming it’s not something common to pre-apelike ancestors, that may suggest it has something to do with the benefit that extra generations have on child-rearing (like caring for a dependent child when mother is off gathering dinner or is downright incapacitated), rather than what otherwise might be the most obvious explanation (human intelligence and the need to pass down complex knowledge).

It could be that human longetivity has it's roots in a common ancestor with other great apes going back tens of millions of years, when all apes (as opposed to all apes but humans—there were no humans at such a time, not even as a genus) had a limited range and tended to live in and around trees (so grandma could just sit up there looking after junior while mom and dad gathered food or warded off predators and all was good as long as there was enough food to go around). While the cost of longetivity might then have gone up as human ancestors started walking around (possibly due to climate change and deforestation, making the ability to walk efficiently rather than swing between trees the optimum mode of transport), it’s not like the elderly would have been capable of overpowering the healthy members of the band to demand a share of food in times of famine (so there would have been a cost to longetivity, but the fit members of the group could easily ensure that cost was born exclusively by those not capable of "chewing the leather" so to speak, and that wouldn’t have even required human intelligence like voting people off the island, it would have been only natural). Which is to say, that there may have been a period of time when longetivity, assuming it evolved in our mostly sedentary tree-dwelling ancestors, became "vestigial" for lack of a better term, but there was nothing driving the outright loss of those genes from the gene pool (external factors and group dynamics could have shortened the lifespan as needed with no change to DNA), and that obviously paid dividends when a band of humans could afford to keep a limited range and live out of one nice cozy cave for a few generations. And, again, if they couldn’t, the ones who either couldn’t keep up or weren’t deemed helpful to survival under exigent circumstances could always be "helped along" to the next world (once our ancestors developed a concept of death and the afterlife, that is).

But of course I am not an evolutionary biologist, I just watch a lot of TV. However it is that humans or their ancestors became capable of outliving their reproductive years by a wide margin, it's important to keep in mind that it’s not like our genes got together and said "you know what? Let’s see if we can make a human live longer, and trim that tale while we’re at it." It’s just that over time, somewhere along the line, incremental mutations in our genes led to changes of physiology that just happened to be beneficial in some circumstances and weren’t costly to maintain in others so that the species, if not to the individual, could become more successful at reproducing. While it might not have increased the number of children any one individual could have before "ageing out," under favorable conditions it may still have increased a particular group's ability to raise children to reproductive maturity themselves, thereby giving that group an advantage over others that lacked that mutation.

Eh—I guess to sum it up, I think you’re right. Help raising the grandchildren.

Last edited by ASL; 06 January 2019 at 05:37 AM.
Reason: Correcting the auto-correct